The present invention relates to thermoplastic molding compositions comprising a mixture of    (a) from 10 to 50% by weight, based on the entirety of components (A), (B), and (C), of a methyl methacrylate polymer as component (A), obtainable via polymerization of a mixture composed of            (a1) from 90 to 100% by weight, based on (A), methyl methacrylate as component (A1), and        (a2) from 0 to 10% by weight, based on (A), of a C1-C8-alkyl ester of acrylic acid as component (A2), and            (b) from 20 to 60% by weight, based on the entirety of components (A), (B), and (C), of a copolymer as component (B), obtainable via polymerization of a mixture composed of            (b1) from 70 to 90% by weight, based on (B), of a vinylaromatic monomer as component (B1), and        (b2) from 10 to 30% by weight, based on (B) of a vinyl cyanide as component (B2), and            (c) from 20 to 70% by weight, based on the entirety of components (A), (B), and (C), of a graft copolymer as component (C), obtainable from            (c1) from 50 to 70% by weight, based on (C), of a core as component (C1), obtainable via polymerization of a monomer mixture composed of                    (c11) from 65 to 89.9% by weight, based on (C1) of a 1,3-diene as component (C11), and            (c12) from 10 to 34.9% by weight, based on (C1), of a vinylaromatic monomer as component (C12), and            (c13) from 0.1 to 5% by weight, based on (C1), of an agglomeration polymer as component (C13), and                        (c2) from 15 to 25% by weight, based on (C), of a first graft shell as component (C2), obtainable via polymerization of a monomer mixture composed of                    (c21) from 30 to 49.9% by weight, based on (C2), of a vinylaromatic monomer as component (C21), and            (c22) from 50 to 69.9% by weight, based on (C2), of a C1-C8-alkyl ester of methacrylic acid and/or of a C1-C8-alkyl ester of acrylic acid as component (C22), and            (c23) from 0.1 to 3% by weight, based on (C2), of a crosslinking monomer as component (C23), and                        (c3) from 15 to 25% by weight, based on (C), of a second graft shell as component (C3), obtainable via polymerization of a monomer mixture composed of        (c31) from 20 to 80% by weight, based on (C3), of a C1-C8-alkyl ester of methacrylic acid as component (C31) and        (c32) from 20 to 80% by weight, based on (C3), of a further monomer as component (C32), and            (d) optionally, amounts of up to 20% by weight, based on the entirety of components (A), (B), and (C), of conventional additives as component (D).
The present invention further relates to processes for producing the thermoplastic molding compositions of the invention, and to the use of these, and to the moldings that can be obtained therefrom.
WO 97/08241 discloses molding compositions which are composed of a hard methyl methacrylate polymer, of a hard vinylaromatic-vinyl cyanide polymer, and of a soft graft copolymer comprising an elastomeric graft core, a first graft shell made of a vinylaromatic-alkyl methacrylate polymer, and a second graft shell made of an alkyl(meth)acrylate polymer. Said molding compositions feature good impact resistance, high flowability, high light transmittance, very low haze, and very little yellow tinge at their edges.
It is known that the mechanical properties of thermoplastic molding compositions can be improved by using elastomeric graft copolymers in which the graft core is composed of comparatively large agglomerated particles which are obtainable by adding an agglomeration polymer during production of the graft cores. Molding compositions of this type based on a very wide variety of plastics matrices are described by way of example in WO 01/83574 and WO 02/10222, as also are processes for producing the same. However, thermoplastic molding compositions of this type comprising graft cores made of comparatively large agglomerated particles usually have impaired optical properties.
WO 2005/075560 and WO 2007/036495 disclose thermoplastic molding compositions based on hard methyl methacrylate polymers, on hard vinylaromatic-vinyl cyanide polymers, and on soft graft copolymers, the mechanical and/or optical properties of which have been further improved. Said graft copolymers comprise a core and a first and a second graft shell, and in the case of WO 2005/075560 the core is characterized by a specific particle size distribution and constitution; factors described as important in WO 2007/036495 include a specific particle size distribution of the core, and also a specific difference between the refractive indices of the graft copolymer and of the matrix.
WO 2005/059029 describes thermoplastic molding compositions based on hard methyl methacrylate polymers, on hard vinylaromatic-vinyl cyanide polymers, and on soft graft copolymers, where these have comparable mechanical, rheological, and optical properties but improved chemicals resistance, for example solvent resistance, or water absorption. Said graft copolymers comprise a core and a first and a second graft shell, where the first graft shell features relatively low vinylaromatic contact and relatively high content of C1-C8-alkyl esters of methacrylic acid.
However, impairment of optical properties often occurs in particular applications, particularly on exposure to moisture over a prolonged period, a particular example being an increase in haze and/or an increase in the level of visible defects known as “specks”.